Learn English – How to use “like” vs “likes” in speech and sentences

For some reason "used to" seems informal to me. I don't think I'd use it (no pun intended) in an academic essay.

But besides that, I don't see a problem with it. Sure, there's ... not exactly an ambiguity, as once you read the complete sentence the meaning should be clear, but what I guess you could call a momentary ambiguity until the reader finishes the sentence. But you could say that about many words. There are lots of words in English that have multiple meanings depending on context. I'm sure we've all had the experience of seeing a word, getting one meaning in our heads, and then reading a little further and having the jarring realization that that was not the intended meaning, so now we have to go back over the sentence and rethink.

So my vote is: In general, don't worry about it and use if freely. But if you're writing a paragraph where you are saying "used to" in the sense of "did in the past" and also "used to" in the sense of "employed for this purpose", you might want to recast the sentence. Like, I think I would avoid writing, "The hammer that Bob used to use to build ..."

The word block can refer to

A lump of wood, stone, or other matter, that obstructs one's way.

(Oxford English Dictionary (OED)).

This refers to a literal lump of wood or stone that you can pick up or remove. According to the OED this usage of block now occurs only in the compound noun stumbling block.

For example, you could say

Watch out for those bricks in the road ahead. They might cause you to stumble or trip. They can be stumbling blocks for anyone who does not walk carefully here.

All this refers to actual physical bricks, which form actual physical stumbling blocks to people who might literally stumble over them. You can actually pick up the bricks (which form the literal stumbling blocks) and clear the path.

The word stumbling block does not normally refer to a physical block or stone.

The link above (Oxford Dictionary online) lists several example sentences. An example off the top of my head is

The growing cost of higher education forms a stumbling block to parents who wish to send several children to university.

Here stumbling block is used figuratively. And that is how it is usually used.

Common synonyms for stumbling block include obstacle and hurdle.

To insist that a stumbling block must be something we can see is wrong.

However, it can refer to something physical:

The pimple on my sweetheart's nose is a stumbling block to me asking her to get married.

but here it's the existence of the pimple that forms a stumbling block or obstacle in the mind of the speaker. It is a physical object that is forming a mental stumbling block to the speaker.

I am not sure what your professor means by

A stumbling block holds back water in a reservoir.

It seems that he is using or trying to use stumbling block in the literal sense, to refer to a literal block or lump of wood or stone or similar that impedes or obstructs the water's progress, so that a reservoir is formed. But stumbling block causes people to stumble, not water or other "inanimate objects."